The present invention relates to a ratio control for a continuously variable transmission, and more particularly to a ratio control for a continuously variable transmission which is designed to improve acceleration performance.
JP-A No. 59-144849 discloses a ratio control system for a continuously variable transmission. According to this known ratio control system, a target value different from a target value for a stable operating state is set during a transient period. More specifically, an engine revolution speed value determined as a function of a throttle opening degree is continuously set as a target value during the stable operating state. The absolute value of a variation in engine revolution speed is used as a criterion for a judgement as to whether a control for a transition period is required or not. According to the known transition control, the target value is kept equal to an engine revolution speed (hereinafter referred to as an old stable engine revolution speed) determined immediately before the change in throttle opening degree for a predetermined period of time, and upon expiration of the predetermined period of time, the target value jumps by a predetermined amount and increases therefrom gradually toward an engine revolution speed (hereinafter referred to as a new stable engine revolution speed) determined for the new throttle opening degree. With this control, a variation in actual engine revolution speed is smoothed against an irregular variation in throttle opening degree. Thus, a drive feeling and a driveability during a transient period are improved.
If this transient control is carried out when a vehicle driver depresses an accelerator pedal to a kick-down position demanding a rapid acceleration, an increase in engine revolution speed is reluctant, so that an initial response is not good and the drive feeling is poor. According to this transient control, since the gradual increase of the target value takes place immediately after the target value has jumped regardless how much the actual value deviates follow the target value, the actual engine revolution speed deviates considerably from the target value after the vehicle driver has depressed the accelerator pedal to the kick-down position at hill climbing, so that a smooth acceleration is not expected and thus a drive feeling is poor.
JP-A No. 59-144850 discloses a ratio control for a continuously variable transmission. According to this known ratio control, with the same throttle opening degree, different target engine revolution speed values are given for different vehicle speeds in order to improve the drive feeling and driveability over a wide range of vehicle speed. Using this target engine revolution speed, at high vehicle speeds, if a vehicle driver depresses an accelerator pedal, an engine revolution speed increases as the vehicle speed increases. At low vehicle speeds, a change in engine revolution speed against a change in throttle opening degree is suppressed.
An object of the present invention is to improve a ratio control for a continuously variable transmission for an automotive vehicle such that an acceleration performance during operation with a heavy load is improved to give a good drive feeling and good driveability.
More specifically, the present invention aims at providing a ratio control for a continuously variable transmission which provides an improved acceleration performance at hill climbing to give a good drive feeling and a good driveability.